Short wave radiation, primarily in the visible range of the spectrum, will penetrate our atmosphere and be reflected or absorbed by the surface of the earth (water or land). If it is reflected it will pass back out of our atmosphere and will NOT warm the earth as part of the Greenhouse Effect.
If the short wave radiation is ABSORBED it must be re-radiated. But, because the earth is so cold compared to the sun, it will be RE-RADIATED as IR, and IR will not pass directly out of the atmosphere (i.e., the wavelength has been converted). This re-radiated IR from the earth's surface will be absorbed by the gases in the atmosphere which, in turn, will re-radiate it also as IR, with some of that radiation being directed back toward the earth.
This temporary "retention" of short wave radiation from the sun (by absorption and re-radiation, thereby changing its wavelength to IR) is the key concept of the Greenhouse Effect.
Note, finally, that the solar radiation initially absorbed by the gases in the atmosphere ALSO WILL CONTRIBUTE to the Greenhouse Effect, because the gases will also re-radiate IR.